The present invention relates to digital signal processing devices such as a digital compressor/limiter and an acoustic-signal-effect-imparting device (effector).
Digital signal processing devices which perform non-linear gain control in accordance with the level of input signal are commonly known, such as a digital compressor/limiter for performing a compressor (compression) or limiter process on high-amplitude input signal in order to prevent undesirable overmodulation from being caused by broadcasting equipment, and an expander/noise gate for, to effectively reproduce music sound in background noises, suppressing low-level noise while performing an expander (expansion) process on low-amplitude signal.
FIG. 16 is a diagram showing an example of an input-output characteristic of such a digital signal processing device. In this diagram, the horizontal axis represents the logarithmic levels of input signals while the vertical axis represents the logarithmic levels of these signals after having passed through the processing device, and the dotted line represents the input-output characteristic of the signals observed when no compression or expansion process is performed by the processing device. Further, FIG. 17A shows logarithmic gains over input signal levels and FIG. 17B shows linear multiplication coefficients over input signal levels.
With such an input-output characteristic, the gain varies as follows depending on the input signal levels:
(1) if an intermediate-level signal (-24 dB to -84 dB) is input, tone volume is increased with an input-output gain of +12 dB;
(2) if a high-level signal (more than -24 dB) is input, compression at a 1/2 compression ratio is performed to prevent clipping at high amplitudes; and
(3) if a low-level signal (less than -84 dB) is input, expansion at a 1/2 expansion ratio is performed, but low-level noise with no signal is suppressed.
However, in the past, complicated calculation or operation circuitry was necessary for achieving such a non-linear gain characteristic.
If a gain characteristic to be achieved is not "non-linear" as referred to in this specification, i.e., if only compression is performed with a substantially constant compression ratio, or if simple logarithmic compression is performed, for instance, then the characteristic can be realized by simple hardware circuitry without complicated operations. For instance, Japanese Patent Application Laid-open No. HEI 3-218109 of a patent application filed by this assignee discloses a compressor which provides a simple compression arrangement by multiplying the input signal itself by a coefficient that is proportional to the level of the input signal. However, the compression characteristic achieved by the disclosed compressor is limited to a relatively simple characteristic, and thus, if gain characteristics as shown in FIGS. 15 and 17 are to be achieved, it is necessary to perform complicated operations using additional circuitry etc.
As mentioned above, the conventional digital signal processing devices employ operation circuitry to achieve nonlinear characteristics. But, the multiplication coefficients change in a complicated manner depending on the varying levels of input signals, and thus, irrespective of whether the operation circuitry employed is analog or digital, the signal processing devices would require complicated and sophisticated operation circuitry. Besides, it was necessary for the user to perform complicated operation of the device in order to change characteristics to be achieved.